The purpose of this research is to identify behavioral and pharmacological variables which alter the relative reinforcing efficacy of cocaine. Through the identification of these variables it will be possible to devise more effective prevention and treatment programs not only for cocaine abuse, but for drug abuse in general. To achieve this purpose, a concurrent schedule choice procedure will be used. In this schedule, the requirement for cocaine reinforcement will increase after each cocaine delivery (progressive ratio schedule), while the requirement for the alternative reinforcer, sucrose, will remain constant. The initial studies will determine the effects of differing concurrent schedules, the effects of increasing the amount of work the subjects must produce per cocaine reinforcement, and the effects of changing the value of the sucrose reinforcer on the relative reinforcing efficacy of cocaine. The focus of the later studies will be to identify variables which shift the preference of the subjects from the cocaine reinforcer to the sucrose reinforcer. The behavioral variables include delay of cocaine reinforcement, the combination of delay and omission of cocaine reinforcement, presenting electric shock at increasing intensities per cocaine reinforcement, and the combination of time out from reinforcement and the presentation of electric shock at some variables on the relationship of cocaine and sucrose reinforcement will also be investigated. Various drugs will be administered pre-session and their ability to shift the preference of the subject will be determined. While some drugs which are known to be reinforcing will be used, the main focus of these studies is the attempt to identify a specific cocaine antagonist.